The present invention relates to a compensator for a liquid crystal display.
Recently the development of the liquid crystal display (hereinafter referred to simply as LCD) has been remarkable, and because of such features thereof as being light, thin and low in power consumption, its position as the leading part of flat panel display has been rendered steady. A supertwisted nematic (simply "STN" hereinafter) is expected to continue its wide use in OA equipment, AV equipment, portable information terminal equipment and the like because of its merit of being low in cost though inferior in image quality and speed to active matrix LCDs, e.g. TFT. Improvements have also been made actively to upgrade STN-LCD to the level of TFT-LCD in image quality and speed while making the most of its low cost merit. Among them, as methods for attaining speed-up there have been developed a method involving improvement of the driving method and a method involving reduction of cell gap. In such high-speed STNs, as a low-molecular liquid crystal for drive cell there is used a liquid crystal wherein the wavelength dispersion of birefringence takes a large value in the range from 1.14 to 1.20 in terms of a dispersion value defined as the ratio of birefringence against 450 nm wavelength light to that against 600 nm light.
Since STN-LCD is basically colored in its display, so for commercialization it is absolutely necessary to use a color compensation film, and it is a stretched film of a polycarbonate for example that is mainly used as present as a retardation film. However, the wavelength dispersion of birefringence of such polycarbonate retardation film is as small as 1.09 in terms of the value defined above and the color compensation of such high speed STN-LCDs has actually been impossible heretofore due to a too large difference in the wavelength dispersion of the two.
With a view to remedying the above-mentioned drawbacks there have been developed retardation films using a material larger in the wavelength dispersion of birefringence such as a polysulfone for example (JP5-224017A, JP5-241021A, JP5-288931A and JP6-94917A). In the case of such polysulfone films, however, although the birefringence dispersion value, which is 1.15, is larger than that of polycarbonate films, a limit is encountered, the dispersion value is a one-point fixed value and not variable, and it is difficult to effect stretching.
On the other hand, as compensating films of a twist structure having a compensating performane superior to that of conventional retardation films, the present inventors have already proposed liquid crystalline polymer type color compensation films produced by fixing a twisted nematic structure (JP3-87720A, JP3-291623A, JP3-294821A and JP4-57017A). Since these compensating films have a twist structure, a more complete color compensation could be attained in comparison with the conventional retardation films not having any twist, but as to the color compensation of high speed STN-LCD, it has been impossible to effect color compensation to a satisfactory extent due to insufficient wavelength dispersion of birefringence.
Having made extensive studies for the purpose of developing a color compensator capable of attaining a sufficiently large wavelength dispersion of birefringence to permit color compensation of high speed STN-LCD and capable of changing the wavelength dispersion value freely, while maintaining the color compensating performance of a liquid crystalline polymer type color compensator having a twisted structure, the present inventors found out that a color compensator consisting principally of a liquid crystalline polyester satisfies such requirements, the said liquid crystalline polyester containing 4-hydroxycinnamic acid units and catechol units as constituent units. Thus, we accomplished the present invention.